<?php
global $var;
$var = $_POST['test'];
if($var == "uno"){
    $sU ="selected";
    echo $sU;
}
if($var == "dos")
{
    $sD = "selected";
    echo $sD;
}
?>
<form id="form" name="form" method="post" action="tupagina.php">
    <select name="test" id="test" size="5" onChange="document.getElementById('form').submit();">
    <option value="uno"<?php echo $sU;?>>Uno1</option>
    <option value="dos"<?php echo $sD;?>>Dos1</option>
</select>
<input type="submit" name="btnAceptar" id="btnAceptar" value="Aceptar" />
</form>
